Hazy Chrome Plate
I am plating a door plate for a lock set made from zinc die cast that has a 90-degree slot into which the door handle is inserted. Why is a haziness visible around the slot area?
Q. I am plating a door plate for a lock set made from zinc die cast that has a 90-degree slot into which the door handle is inserted. A haziness is visible around the slot area. I’ve thoroughly examined the copper strike, the copper pyrophosphate and the nickel plating baths prior to the chromium plating bath, but have had no success in solving the problem. Do you have any suggestions? —C.J.
A. There are many possible reasons for the haze that you see around the cutout area of the plate, but before addressing possible ways to solve your problem, let’s discuss the fundamental issue regarding that cutout: current density in and around it. Intuitively, you might think that there is no difference in the current density, but you will have a slightly higher current density in the cutout area because of the slot’s edges. If the current density is slightly higher in the slot area, you would expect more chrome to be deposited there, and if there are not enough chromium ions available in the surrounding solution, you may see the haze you describe.
So what are some of the things that might help solve the problem? To start with, do you see any evidence of this haze after the bright-nickel plating step? If no, then you must look at your chromium plating bath. Start by looking at the temperature of the rinse tank prior to the chromium plating bath. Warmer rinses generally do a better job of clearing the surface of the part. In this case, it also may warm the parts slightly so that when they enter the chromium bath they reach bath temperature more quickly. Before starting the chromium plating step, you should allow the parts to reach temperature equilibrium in the plating bath.
Your temperature-current density ratio also may need adjusting. Raising the temperature of your plating bath a few degrees may be all that is needed. Another possibility is to reduce your average for a given load by 5 to 10 percent. You should also check out the chemistry of your chromium plating bath to verify that it is within the recommended chemical concentrations.
I am sending you a copy of the "Chromium Tank Doctor," a troubleshooting chart for classic hexavalent chrome plating. You may find a few hints in it that will help you with your chromium plating.